Evidence for monoclinic crystal structure and negative thermal expansion below magnetic transition temperature in Pb (Fe12 Nb12) O3

Abstract

The existing controversy about the room temperature structure of multiferroic Pb (Fe12 Nb12) O3 is settled using synchrotron powder x-ray diffraction data. Results of Rietveld refinements in the temperature range of 300-12 K reveal that the structure remains monoclinic in the Cm space group down to 12 K, but the lattice parameters show anomalies at the magnetic transition temperature (TN) due to spin-lattice coupling. The lattice volume exhibits negative thermal expansion behavior, with α=-4.64× 10-6 K-1, below TN.